This project is concerned with the possible trophic action from the target organ to the innervating neurons in the mammal. When the activity of a skeletal muscle was reduced by partial denervation in the cat, the motoneurons whose axons had been left intact upon partial denervation showed alterations in their properties. Since these intact motoneurons were not exposed to any injury in their axons, it is likely that motoneuron properties may depend partly on the conditions of the innervated muscle. When the peripheral branches of sensory neurons of the dorsal root ganglia were sectioned, the sensory neurons changed their properties. In contrast, section of the central branches did not produce any change in the properties of sensory neurons. It is suggested that normal neuron properties are maintained by trophic signals arising from the peripheral target organ.